Background: Despite the success of HAART in the treatment of individuals with HIV-1, such therapeutic combinations have limitations. The application of HAART with a therapeutic vaccine has the potential to improve HIV-1 specific immunity. Dendritic cells (DCs) are likely to play a critical role in the induction of cellular immune responses to vaccine antigens. We have shown that whole Saccharomyces cerevisiae yeast recombinant for a model tumor antigen induced CD8⁺ T-cell mediated tumor protection in vaccinated mice and induced murine DC maturation and IL-12 production. To determine the potential for using recombinant yeast as an HIV-1 vaccine, we investigated the interactions between yeast and human DCs.

Methods: Monocyte-derived DCs (MDDCs), whole PBMCs, and purified blood CD11c⁺ myeloid DCs (mDCs) and CD11c^- plasmacytoid DCs (pDCs) from HAART treated HIV-1 infected subjects and uninfected controls were cultured 24–48 hrs with media alone, recombinant yeast, or LPS. Flow cytometry was used to evaluate the resulting DC immunophenotype. Cytokine production was assessed on frozen cell culture supernatants using standard ELISA kits.

Results: MDDCs from both HIV-1-infected and uninfected donors cultured with yeast displayed increases in CD40, CD80, CD83, and MHC Class II expression consistent with maturation and produced significant amounts of TNF-γ, IL-12, and IL-10. Yeast also induced phenotypic maturation of both mDCs and pDCs in whole PBMCs from HIV-1-infected subjects and controls. PBMC cultured with yeast produced significant levels of TNF-γ but more variable levels of IL-10 and IL-12. Experiments using purified blood DCs showed a direct maturation effect of yeast only on the mDC subset. Cytokine production by purified blood DC subsets in response to yeast is currently under investigation.

Conclusions: Phenotypic maturation of cultured MDDCs and both subsets of blood DCs in total PBMCs from HIV-infected and uninfected donors was observed following stimulation with yeast. Yeast-pulsed MDDCs produced a predominance of inflammatory cytokines, without a clear Th1- or Th2-biasing profile. Culture of whole PBMCs with yeast had a similar cytokine profile. These results suggest that recombinant yeast vaccine has a potent adjuvant effect on human DCs that may allow for the effective induction of cellular immune responses in either the prophylactic or therapeutic vaccine settings.